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PERLDATA(1) Perl Programmers Reference Guide PERLDATA(1)
NAME
perldata - Perl data types
DESCRIPTION
Variable names
Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as "hashes". A scalar is a single
string (of any size, limited only by the available memory), number, or
a reference to something (which will be discussed in perlref). Normal
arrays are ordered lists of scalars indexed by number, starting with 0.
Hashes are unordered collections of scalar values indexed by their
associated string key.
Values are usually referred to by name, or through a named reference.
The first character of the name tells you to what sort of data struc-
ture it refers. The rest of the name tells you the particular value to
which it refers. Usually this name is a single identifier, that is, a
string beginning with a letter or underscore, and containing letters,
underscores, and digits. In some cases, it may be a chain of identi-
fiers, separated by "::" (or by the slightly archaic "'"); all but the
last are interpreted as names of packages, to locate the namespace in
which to look up the final identifier (see "Packages" in perlmod for
details). It's possible to substitute for a simple identifier, an
expression that produces a reference to the value at runtime. This is
described in more detail below and in perlref.
Perl also has its own built-in variables whose names don't follow these
rules. They have strange names so they don't accidentally collide with
one of your normal variables. Strings that match parenthesized parts
of a regular expression are saved under names containing only digits
after the "$" (see perlop and perlre). In addition, several special
variables that provide windows into the inner working of Perl have
names containing punctuation characters and control characters. These
are documented in perlvar.
Scalar values are always named with '$', even when referring to a
scalar that is part of an array or a hash. The '$' symbol works seman-
tically like the English word "the" in that it indicates a single value
is expected.
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@',
which works much like the word "these" or "those" does in English, in
that it indicates multiple values are expected.
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
Entire hashes are denoted by '%':
%days # (key1, val1, key2, val2 ...)
In addition, subroutines are named with an initial '&', though this is
optional when unambiguous, just as the word "do" is often redundant in
English. Symbol table entries can be named with an initial '*', but
you don't really care about that yet (if ever :-).
Every variable type has its own namespace, as do several non-variable
identifiers. This means that you can, without fear of conflict, use
the same name for a scalar variable, an array, or a hash--or, for that
matter, for a filehandle, a directory handle, a subroutine name, a for-
mat name, or a label. This means that $foo and @foo are two different
variables. It also means that $foo[1] is a part of @foo, not a part of
$foo. This may seem a bit weird, but that's okay, because it is weird.
Because variable references always start with '$', '@', or '%', the
"reserved" words aren't in fact reserved with respect to variable
names. They are reserved with respect to labels and filehandles, how-
ever, which don't have an initial special character. You can't have a
filehandle named "log", for instance. Hint: you could say
"open(LOG,'logfile')" rather than "open(log,'logfile')". Using upper-
case filehandles also improves readability and protects you from con-
flict with future reserved words. Case is significant--"FOO", "Foo",
and "foo" are all different names. Names that start with a letter or
underscore may also contain digits and underscores.
It is possible to replace such an alphanumeric name with an expression
that returns a reference to the appropriate type. For a description of
this, see perlref.
Names that start with a digit may contain only more digits. Names that
do not start with a letter, underscore, digit or a caret (i.e. a con-
trol character) are limited to one character, e.g., $% or $$. (Most
of these one character names have a predefined significance to Perl.
For instance, $$ is the current process id.)
Context
The interpretation of operations and values in Perl sometimes depends
on the requirements of the context around the operation or value.
There are two major contexts: list and scalar. Certain operations
return list values in contexts wanting a list, and scalar values other-
wise. If this is true of an operation it will be mentioned in the doc-
umentation for that operation. In other words, Perl overloads certain
operations based on whether the expected return value is singular or
plural. Some words in English work this way, like "fish" and "sheep".
In a reciprocal fashion, an operation provides either a scalar or a
list context to each of its arguments. For example, if you say
int( )
the integer operation provides scalar context for the <> operator,
which responds by reading one line from STDIN and passing it back to
the integer operation, which will then find the integer value of that
line and return that. If, on the other hand, you say
sort( )
then the sort operation provides list context for <>, which will pro-
ceed to read every line available up to the end of file, and pass that
list of lines back to the sort routine, which will then sort those
lines and return them as a list to whatever the context of the sort
was.
Assignment is a little bit special in that it uses its left argument to
determine the context for the right argument. Assignment to a scalar
evaluates the right-hand side in scalar context, while assignment to an
array or hash evaluates the righthand side in list context. Assignment
to a list (or slice, which is just a list anyway) also evaluates the
righthand side in list context.
When you use the "use warnings" pragma or Perl's -w command-line
option, you may see warnings about useless uses of constants or func-
tions in "void context". Void context just means the value has been
discarded, such as a statement containing only ""fred";" or "getp-
wuid(0);". It still counts as scalar context for functions that care
whether or not they're being called in list context.
User-defined subroutines may choose to care whether they are being
called in a void, scalar, or list context. Most subroutines do not
need to bother, though. That's because both scalars and lists are
automatically interpolated into lists. See "wantarray" in perlfunc for
how you would dynamically discern your function's calling context.
Scalar values
All data in Perl is a scalar, an array of scalars, or a hash of
scalars. A scalar may contain one single value in any of three differ-
ent flavors: a number, a string, or a reference. In general, conver-
sion from one form to another is transparent. Although a scalar may
not directly hold multiple values, it may contain a reference to an
array or hash which in turn contains multiple values.
Scalars aren't necessarily one thing or another. There's no place to
declare a scalar variable to be of type "string", type "number", type
"reference", or anything else. Because of the automatic conversion of
scalars, operations that return scalars don't need to care (and in
fact, cannot care) whether their caller is looking for a string, a num-
ber, or a reference. Perl is a contextually polymorphic language whose
scalars can be strings, numbers, or references (which includes
objects). Although strings and numbers are considered pretty much the
same thing for nearly all purposes, references are strongly-typed,
uncastable pointers with builtin reference-counting and destructor
invocation.
A scalar value is interpreted as TRUE in the Boolean sense if it is not
the null string or the number 0 (or its string equivalent, "0"). The
Boolean context is just a special kind of scalar context where no con-
version to a string or a number is ever performed.
There are actually two varieties of null strings (sometimes referred to
as "empty" strings), a defined one and an undefined one. The defined
version is just a string of length zero, such as "". The undefined
version is the value that indicates that there is no real value for
something, such as when there was an error, or at end of file, or when
you refer to an uninitialized variable or element of an array or hash.
Although in early versions of Perl, an undefined scalar could become
defined when first used in a place expecting a defined value, this no
longer happens except for rare cases of autovivification as explained
in perlref. You can use the defined() operator to determine whether a
scalar value is defined (this has no meaning on arrays or hashes), and
the undef() operator to produce an undefined value.
To find out whether a given string is a valid non-zero number, it's
sometimes enough to test it against both numeric 0 and also lexical "0"
(although this will cause noises if warnings are on). That's because
strings that aren't numbers count as 0, just as they do in awk:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
That method may be best because otherwise you won't treat IEEE nota-
tions like "NaN" or "Infinity" properly. At other times, you might
prefer to determine whether string data can be used numerically by
calling the POSIX::strtod() function or by inspecting your string with
a regular expression (as documented in perlre).
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # rejects -3
warn "not an integer" unless /^-?\d+$/; # rejects +3
warn "not an integer" unless /^[+-]?\d+$/;
warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
The length of an array is a scalar value. You may find the length of
array @days by evaluating $#days, as in csh. However, this isn't the
length of the array; it's the subscript of the last element, which is a
different value since there is ordinarily a 0th element. Assigning to
$#days actually changes the length of the array. Shortening an array
this way destroys intervening values. Lengthening an array that was
previously shortened does not recover values that were in those ele-
ments. (It used to do so in Perl 4, but we had to break this to make
sure destructors were called when expected.)
You can also gain some minuscule measure of efficiency by pre-extending
an array that is going to get big. You can also extend an array by
assigning to an element that is off the end of the array. You can
truncate an array down to nothing by assigning the null list () to it.
The following are equivalent:
@whatever = ();
$#whatever = -1;
If you evaluate an array in scalar context, it returns the length of
the array. (Note that this is not true of lists, which return the last
value, like the C comma operator, nor of built-in functions, which
return whatever they feel like returning.) The following is always
true:
scalar(@whatever) == $#whatever - $[ + 1;
Version 5 of Perl changed the semantics of $[: files that don't set the
value of $[ no longer need to worry about whether another file changed
its value. (In other words, use of $[ is deprecated.) So in general
you can assume that
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so as to leave
nothing to doubt:
$element_count = scalar(@whatever);
If you evaluate a hash in scalar context, it returns false if the hash
is empty. If there are any key/value pairs, it returns true; more pre-
cisely, the value returned is a string consisting of the number of used
buckets and the number of allocated buckets, separated by a slash.
This is pretty much useful only to find out whether Perl's internal
hashing algorithm is performing poorly on your data set. For example,
you stick 10,000 things in a hash, but evaluating %HASH in scalar con-
text reveals "1/16", which means only one out of sixteen buckets has
been touched, and presumably contains all 10,000 of your items. This
isn't supposed to happen.
You can preallocate space for a hash by assigning to the keys() func-
tion. This rounds up the allocated buckets to the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
Scalar value constructors
Numeric literals are specified in any of the following floating point
or integer formats:
12345
12345.67
.23E-10 # a very small number
3.14_15_92 # a very important number
4_294_967_296 # underscore for legibility
0xff # hex
0xdead_beef # more hex
0377 # octal (only numbers, begins with 0)
0b011011 # binary
You are allowed to use underscores (underbars) in numeric literals
between digits for legibility. You could, for example, group binary
digits by threes (as for a Unix-style mode argument such as
0b110_100_100) or by fours (to represent nibbles, as in 0b1010_0110) or
in other groups.
String literals are usually delimited by either single or double
quotes. They work much like quotes in the standard Unix shells: dou-
ble-quoted string literals are subject to backslash and variable sub-
stitution; single-quoted strings are not (except for "\'" and "\\").
The usual C-style backslash rules apply for making characters such as
newline, tab, etc., as well as some more exotic forms. See "Quote and
Quote-like Operators" in perlop for a list.
Hexadecimal, octal, or binary, representations in string literals (e.g.
'0xff') are not automatically converted to their integer representa-
tion. The hex() and oct() functions make these conversions for you.
See "hex" in perlfunc and "oct" in perlfunc for more details.
You can also embed newlines directly in your strings, i.e., they can
end on a different line than they begin. This is nice, but if you for-
get your trailing quote, the error will not be reported until Perl
finds another line containing the quote character, which may be much
further on in the script. Variable substitution inside strings is lim-
ited to scalar variables, arrays, and array or hash slices. (In other
words, names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out "The
price is $100."
$Price = '$100'; # not interpolated
print "The price is $Price.\n"; # interpolated
There is no double interpolation in Perl, so the $100 is left as is.
As in some shells, you can enclose the variable name in braces to dis-
ambiguate it from following alphanumerics (and underscores). You must
also do this when interpolating a variable into a string to separate
the variable name from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
Without the braces, Perl would have looked for a $whospeak, a $who::0,
and a $who's variable. The last two would be the $0 and the $s vari-
ables in the (presumably) non-existent package "who".
In fact, an identifier within such curlies is forced to be a string, as
is any simple identifier within a hash subscript. Neither need quot-
ing. Our earlier example, $days{'Feb'} can be written as $days{Feb}
and the quotes will be assumed automatically. But anything more com-
plicated in the subscript will be interpreted as an expression. This
means for example that "$version{2.0}++" is equivalent to "$ver-
sion{2}++", not to "$version{'2.0'}++".
Version Strings
Note: Version Strings (v-strings) have been deprecated. They will not
be available after Perl 5.8. The marginal benefits of v-strings were
greatly outweighed by the potential for Surprise and Confusion.
A literal of the form "v1.20.300.4000" is parsed as a string composed
of characters with the specified ordinals. This form, known as
v-strings, provides an alternative, more readable way to construct
strings, rather than use the somewhat less readable interpolation form
"\x{1}\x{14}\x{12c}\x{fa0}". This is useful for representing Unicode
strings, and for comparing version "numbers" using the string compari-
son operators, "cmp", "gt", "lt" etc. If there are two or more dots in
the literal, the leading "v" may be omitted.
print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both "require" and "use" for doing a ver-
sion check. The $^V special variable also contains the running Perl
interpreter's version in this form. See "$^V" in perlvar. Note that
using the v-strings for IPv4 addresses is not portable unless you also
use the inet_aton()/inet_ntoa() routines of the Socket package.
Note that since Perl 5.8.1 the single-number v-strings (like "v65") are
not v-strings before the "=>" operator (which is usually used to sepa-
rate a hash key from a hash value), instead they are interpreted as
literal strings ('v65'). They were v-strings from Perl 5.6.0 to Perl
5.8.0, but that caused more confusion and breakage than good. Multi-
number v-strings like "v65.66" and 65.66.67 continue to be v-strings
always.
Special Literals
The special literals __FILE__, __LINE__, and __PACKAGE__ represent the
current filename, line number, and package name at that point in your
program. They may be used only as separate tokens; they will not be
interpolated into strings. If there is no current package (due to an
empty "package;" directive), __PACKAGE__ is the undefined value.
The two control characters ^D and ^Z, and the tokens __END__ and
__DATA__ may be used to indicate the logical end of the script before
the actual end of file. Any following text is ignored.
Text after __DATA__ but may be read via the filehandle "PACK-
NAME::DATA", where "PACKNAME" is the package that was current when the
__DATA__ token was encountered. The filehandle is left open pointing
to the contents after __DATA__. It is the program's responsibility to
"close DATA" when it is done reading from it. For compatibility with
older scripts written before __DATA__ was introduced, __END__ behaves
like __DATA__ in the toplevel script (but not in files loaded with
"require" or "do") and leaves the remaining contents of the file acces-
sible via "main::DATA".
See SelfLoader for more description of __DATA__, and an example of its
use. Note that you cannot read from the DATA filehandle in a BEGIN
block: the BEGIN block is executed as soon as it is seen (during compi-
lation), at which point the corresponding __DATA__ (or __END__) token
has not yet been seen.
Barewords
A word that has no other interpretation in the grammar will be treated
as if it were a quoted string. These are known as "barewords". As
with filehandles and labels, a bareword that consists entirely of low-
ercase letters risks conflict with future reserved words, and if you
use the "use warnings" pragma or the -w switch, Perl will warn you
about any such words. Some people may wish to outlaw barewords
entirely. If you say
use strict 'subs';
then any bareword that would NOT be interpreted as a subroutine call
produces a compile-time error instead. The restriction lasts to the
end of the enclosing block. An inner block may countermand this by
saying "no strict 'subs'".
Array Joining Delimiter
Arrays and slices are interpolated into double-quoted strings by join-
ing the elements with the delimiter specified in the $" variable
($LIST_SEPARATOR if "use English;" is specified), space by default.
The following are equivalent:
$temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish substitution)
there is an unfortunate ambiguity: Is "/$foo[bar]/" to be interpreted
as "/${foo}[bar]/" (where "[bar]" is a character class for the regular
expression) or as "/${foo[bar]}/" (where "[bar]" is the subscript to
array @foo)? If @foo doesn't otherwise exist, then it's obviously a
character class. If @foo exists, Perl takes a good guess about
"[bar]", and is almost always right. If it does guess wrong, or if
you're just plain paranoid, you can force the correct interpretation
with curly braces as above.
If you're looking for the information on how to use here-documents,
which used to be here, that's been moved to "Quote and Quote-like Oper-
ators" in perlop.
List value constructors
List values are denoted by separating individual values by commas (and
enclosing the list in parentheses where precedence requires it):
(LIST)
In a context not requiring a list value, the value of what appears to
be a list literal is simply the value of the final element, as with the
C comma operator. For example,
@foo = ('cc', '-E', $bar);
assigns the entire list value to array @foo, but
$foo = ('cc', '-E', $bar);
assigns the value of variable $bar to the scalar variable $foo. Note
that the value of an actual array in scalar context is the length of
the array; the following assigns the value 3 to $foo:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of a list
literal, so that you can say:
@foo = (
1,
2,
3,
);
To use a here-document to assign an array, one line per element, you
might use an approach like this:
@sauces = <NOT interpolate, see perlref.
The null list is represented by (). Interpolating it in a list has no
effect. Thus ((),(),()) is equivalent to (). Similarly, interpolating
an array with no elements is the same as if no array had been interpo-
lated at that point.
This interpolation combines with the facts that the opening and closing
parentheses are optional (except when necessary for precedence) and
lists may end with an optional comma to mean that multiple commas
within lists are legal syntax. The list "1,,3" is a concatenation of
two lists, "1," and 3, the first of which ends with that optional
comma. "1,,3" is "(1,),(3)" is "1,3" (And similarly for "1,,,3" is
"(1,),(,),3" is "1,3" and so on.) Not that we'd advise you to use this
obfuscation.
A list value may also be subscripted like a normal array. You must put
the list in parentheses to avoid ambiguity. For example:
# Stat returns list value.
$time = (stat($file))[8];
# SYNTAX ERROR HERE.
$time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
# Find a hex digit.
$hexdigit = ('a','b','c','d','e','f')[$digit-10];
# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];
Lists may be assigned to only when each element of the list is itself
legal to assign to:
($a, $b, $c) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
An exception to this is that you may assign to "undef" in a list. This
is useful for throwing away some of the return values of a function:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
List assignment in scalar context returns the number of elements pro-
duced by the expression on the right side of the assignment:
$x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
$x = (($foo,$bar) = f()); # set $x to f()'s return count
This is handy when you want to do a list assignment in a Boolean con-
text, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as FALSE.
It's also the source of a useful idiom for executing a function or per-
forming an operation in list context and then counting the number of
return values, by assigning to an empty list and then using that
assignment in scalar context. For example, this code:
$count = () = $string =~ /\d+/g;
will place into $count the number of digit groups found in $string.
This happens because the pattern match is in list context (since it is
being assigned to the empty list), and will therefore return a list of
all matching parts of the string. The list assignment in scalar context
will translate that into the number of elements (here, the number of
times the pattern matched) and assign that to $count. Note that simply
using
$count = $string =~ /\d+/g;
would not have worked, since a pattern match in scalar context will
only return true or false, rather than a count of matches.
The final element of a list assignment may be an array or a hash:
($a, $b, @rest) = split;
my($a, $b, %rest) = @_;
You can actually put an array or hash anywhere in the list, but the
first one in the list will soak up all the values, and anything after
it will become undefined. This may be useful in a my() or local().
A hash can be initialized using a literal list holding pairs of items
to be interpreted as a key and a value:
# same as map assignment above
%map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
While literal lists and named arrays are often interchangeable, that's
not the case for hashes. Just because you can subscript a list value
like a normal array does not mean that you can subscript a list value
as a hash. Likewise, hashes included as parts of other lists (includ-
ing parameters lists and return lists from functions) always flatten
out into key/value pairs. That's why it's good to use references some-
times.
It is often more readable to use the "=>" operator between key/value
pairs. The "=>" operator is mostly just a more visually distinctive
synonym for a comma, but it also arranges for its left-hand operand to
be interpreted as a string -- if it's a bareword that would be a legal
simple identifier ("=>" doesn't quote compound identifiers, that con-
tain double colons). This makes it nice for initializing hashes:
%map = (
red => 0x00f,
blue => 0x0f0,
green => 0xf00,
);
or for initializing hash references to be used as records:
$rec = {
witch => 'Mable the Merciless',
cat => 'Fluffy the Ferocious',
date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
$field = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => \%labels
);
Note that just because a hash is initialized in that order doesn't mean
that it comes out in that order. See "sort" in perlfunc for examples
of how to arrange for an output ordering.
Subscripts
An array is subscripted by specifying a dollar sign ("$"), then the
name of the array (without the leading "@"), then the subscript inside
square brackets. For example:
@myarray = (5, 50, 500, 5000);
print "Element Number 2 is", $myarray[2], "\n";
The array indices start with 0. A negative subscript retrieves its
value from the end. In our example, $myarray[-1] would have been 5000,
and $myarray[-2] would have been 500.
Hash subscripts are similar, only instead of square brackets curly
brackets are used. For example:
%scientists =
(
"Newton" => "Isaac",
"Einstein" => "Albert",
"Darwin" => "Charles",
"Feynman" => "Richard",
);
print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
Slices
A common way to access an array or a hash is one scalar element at a
time. You can also subscript a list to get a single element from it.
$whoami = $ENV{"USER"}; # one element from the hash
$parent = $ISA[0]; # one element from the array
$dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a hash simul-
taneously using a list of subscripts. It's more convenient than writ-
ing out the individual elements as a list of separate scalar values.
($him, $her) = @folks[0,-1]; # array slice
@them = @folks[0 .. 3]; # array slice
($who, $home) = @ENV{"USER", "HOME"}; # hash slice
($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to an
array or hash slice.
@days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
= (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
($colors{'red'}, $colors{'blue'}, $colors{'green'})
= (0xff0000, 0x0000ff, 0x00ff00);
($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
Since changing a slice changes the original array or hash that it's
slicing, a "foreach" construct will alter some--or even all--of the
values of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{qw[key1 key2]}) {
s/^\s+//; # trim leading whitespace
s/\s+$//; # trim trailing whitespace
s/(\w+)/\u\L$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
@c = (0,1)[2,3]; # @c has no elements
But:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a null list is
returned:
while ( ($home, $user) = (getpwent)[7,0]) {
printf "%-8s %s\n", $user, $home;
}
As noted earlier in this document, the scalar sense of list assignment
is the number of elements on the right-hand side of the assignment.
The null list contains no elements, so when the password file is
exhausted, the result is 0, not 2.
If you're confused about why you use an '@' there on a hash slice
instead of a '%', think of it like this. The type of bracket (square
or curly) governs whether it's an array or a hash being looked at. On
the other hand, the leading symbol ('$' or '@') on the array or hash
indicates whether you are getting back a singular value (a scalar) or a
plural one (a list).
Typeglobs and Filehandles
Perl uses an internal type called a typeglob to hold an entire symbol
table entry. The type prefix of a typeglob is a "*", because it repre-
sents all types. This used to be the preferred way to pass arrays and
hashes by reference into a function, but now that we have real refer-
ences, this is seldom needed.
The main use of typeglobs in modern Perl is create symbol table
aliases. This assignment:
*this = *that;
makes $this an alias for $that, @this an alias for @that, %this an
alias for %that, &this an alias for &that, etc. Much safer is to use a
reference. This:
local *Here::blue = \$There::green;
temporarily makes $Here::blue an alias for $There::green, but doesn't
make @Here::blue an alias for @There::green, or %Here::blue an alias
for %There::green, etc. See "Symbol Tables" in perlmod for more exam-
ples of this. Strange though this may seem, this is the basis for the
whole module import/export system.
Another use for typeglobs is to pass filehandles into a function or to
create new filehandles. If you need to use a typeglob to save away a
filehandle, do it this way:
$fh = *STDOUT;
or perhaps as a real reference, like this:
$fh = \*STDOUT;
See perlsub for examples of using these as indirect filehandles in
functions.
Typeglobs are also a way to create a local filehandle using the local()
operator. These last until their block is exited, but may be passed
back. For example:
sub newopen {
my $path = shift;
local *FH; # not my!
open (FH, $path) or return undef;
return *FH;
}
$fh = newopen('/etc/passwd');
Now that we have the *foo{THING} notation, typeglobs aren't used as
much for filehandle manipulations, although they're still needed to
pass brand new file and directory handles into or out of functions.
That's because *HANDLE{IO} only works if HANDLE has already been used
as a handle. In other words, *FH must be used to create new symbol ta-
ble entries; *foo{THING} cannot. When in doubt, use *FH.
All functions that are capable of creating filehandles (open(),
opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
automatically create an anonymous filehandle if the handle passed to
them is an uninitialized scalar variable. This allows the constructs
such as "open(my $fh, ...)" and "open(local $fh,...)" to be used to
create filehandles that will conveniently be closed automatically when
the scope ends, provided there are no other references to them. This
largely eliminates the need for typeglobs when opening filehandles that
must be passed around, as in the following example:
sub myopen {
open my $fh, "@_"
or die "Can't open '@_': $!";
return $fh;
}
{
my $f = myopen(";
# $f implicitly closed here
}
Note that if an initialized scalar variable is used instead the result
is different: "my $fh='zzz'; open($fh, ...)" is equivalent to "open(
*{'zzz'}, ...)". "use strict 'refs'" forbids such practice.
Another way to create anonymous filehandles is with the Symbol module
or with the IO::Handle module and its ilk. These modules have the
advantage of not hiding different types of the same name during the
local(). See the bottom of "open()" in perlfunc for an example.
SEE ALSO
See perlvar for a description of Perl's built-in variables and a dis-
cussion of legal variable names. See perlref, perlsub, and "Symbol
Tables" in perlmod for more discussion on typeglobs and the *foo{THING}
syntax.
perl v5.8.8 2006-01-07 PERLDATA(1)
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